The present invention relates to an optical pickup apparatus for reading informational signals written on, for example, a magneto-optical disc.
So-called magneto-optical discs comprising a signal recording layer which is perpendicularly magnetizable so that informational signals can be written in accordance with differences in the direction of magnetization of the signal recording layer have heretofore been proposed as recording media for recording informational thereon.
In order to write the informational signals on the magneto-optical disc, only small domains of the signal recording layer is heated to a temperature higher than a Curie temperature by means such irradiating the domains of the signal recording layer with a converged laser beam to cause the coercive force to disappear and an external magnetic field is applied to the small domains to change the magnetization of the small domains. In such a manner, writing of informational signals is performed by changing the magnetization direction in the signal recording layer for each small domain.
In order to read the informational signals which have been written in the magneto-optical disc, a flux of light having a signal polarization direction such as a laser beam is converged and impinged upon the signal recording layer to detect the polarization direction of the reflected light of the flux of light. The flux of light changes the polarization direction depending upon the direction of magnetization due to so-called Kerr effect when it is reflected on the signal recording layer. Accordingly, the differences is magnetization direction of the signal recording layer can be detected by detecting the polarization direction of the reflected light of the flux.
Therefore, in order to read the informational signals from magneto-optical disc, an optical pickup apparatus is used which is formed so that the signal layer 1a of the magneto-optical disc 1 is irradiated with the laser beam and the light reflected from the signal recording layer 1a is detected as shown in FIG. 9.
The optical pickup apparatus has a light source 102 such as semiconductor laser and is formed so that a flux of light emitted from the light source 102 is converged and is impinged upon the surface of the signal recording layer 1a via optical devices such as a collimator lens 103, a beam splitter 104 and an objective lens 105.
The optical pickup apparatus is also formed so that the reflected light of the flux of light which is incident upon the surface of the signal recording layer 1a is introduced to a detecting optical system via the objective lens 105 and the reflection surface 104a of the beam splitter 104 to detect the polarization direction of the reflected light.
The detecting optical system in the optical pickup apparatus may includes a detecting lens 107 for converging the flux of light incident upon the detecting optical system, first and second photo-detecting element 109 and 110, and a prism 108 disposed in an optical path between the detecting lens 107 and each of the photo-detecting elements 109 and 110.
The prism 108 is formed into a prism having a substantially trapezoidal cross section by bonding a parallelogrammatic prism 108a to a triangular prism 108b each other at a polarization reflection surface 108c. The flux of light which has transmitted through the detecting lens 107 is firstly substantially normally incident upon one side of the parallelogrammatic prism 108a constituting an upper bottom portion of the prism 108 and is then incliningly incident upon the polarization reflection surface 108c. A part of the flux of light is reflected by the polarization reflection surface 108c depending upon the polarization direction and the remaining flux will be transmitted through the surface 108c. The ratio of the reflected light to the transmitted light changes depending upon the polarization direction of the flux of light.
The flux of light which has been transmitted through the polarization reflection surface 108c is detected as a first beam spot S.sub.1 by the first photo-detecting element 109 secured to the triangular prism 108b forming a part of the lower bottom portion of the prism 108. On the other hand, the flux of light reflected by the polarization reflection surface 108c is reflected by the slanted side of the parallelogrammatic prism 108a forming the slanted side of the prism 108 is detected as a second beam spot S.sub.2 by the second photo-detecting element 110 secured to the parallelogrammatic prism 108a forming the other portion of the lower bottom portion of the prism 108.
The differential signal between the photo detection signals outputted from the photo-detecting elements 109 and 110 which detect the fluxes of light separated by the polarization reflection surface 108c, that is, the difference between the amounts of light of the beam spots S.sub.1 and S.sub.2 will become a read signal of the information signal written in the magneto-optical disc 1.
Each of the photo-detecting elements 109 and 110 comprises three parallel photo-receiving areas as shown in FIG. 10. That is, the first photo-detecting element 109 comprises first to third photo-receiving areas 109a, 109b and 109c. The second photo-detecting element 110 comprises fourth to sixth photo-receiving areas 110a, 110b and 110c. The convergence condition of the flux of light on the photo-receiving elements 109 and 110 is changed depending upon the focus error of the flux of light focussed on the surface of the signal recording layer 1a of the magneto-optical disc 1.
Therefore, so-called focus error signal representative of the amount of the focus error on the surface of the signal recording layer 1a of the magnetic-optical disc 1 can be obtained by so-called internal and external differential method in which a signal representative of a difference between the photo detection outputs from the photo-receiving areas 109b and 110b in the central sides of the photo-detecting elements 109 and 110 and the photo detection outputs from the photo-receiving areas 109a, 109c, 110a, 109c on the opposite sides of the photo-detecting elements 109 and 110. That is, if the photo detection outputs from the first to sixth photo-receiving areas 109a, 109b, 109c, 110a, 110b and 110c are represented as A, B, C, D, E and F respectively, the focus error signal Fe can be determined by the formula as follows: EQU Fe=(B-(A+C))-(E-(D+F))
In an optical pickup apparatus comprising a prism 108 having the above-mentioned polarization reflection surface 108c, the first and second photo-detecting elements 109 and 110 have boundary lines between photo-receiving areas which are inclined with each other. It is hard to keep the high precision of the securing position of each photo-detecting elements 109 and 110 since the photo-detecting elements 109 and 110 prism 108 by means such as bonding. The beam spots S.sub.1 and S.sub.2 are formed on the photo-detecting elements 109 and 110, respectively by securing the photo-detecting elements 109 and 110 to the prism 108 so that they are inclined with each other and by adjusting the position of the prism 108.
Accordingly, if a so-called tracking error occurs in which the flux of light incident upon the signal recording layer 1a is deviated relative to a recording track formed on the signal recording layer 1a of the magneto-optical disc 1 in the optical pickup apparatus, a so-called focus offset occurs since the beam spots S.sub.1 and S.sub.2 will be moved in a direction inclined with respect to the boundary lines of the photo-receiving areas constituting each photo-detecting element 109 and 110. The condition on which the focus offset occurs is defined as a condition that a focus error signal representing as if a focus error occurred is outputted although the focus error does not actually occur on the surface of the signal recording layer 1a.
In order to prevent the focus offset from occurring, an option is possible that the photo detecting elements 109 and 110 are mounted by adjusting the position thereof to make parallel the boundary lines between the photo-receiving areas forming the photo-detecting elements 109 and 110. However, the structure of the thus formed optical pickup apparatus becomes more complicated in structure and its assembly and manufacturing becomes more hard.
The present invention was proposed in view of the above-mentioned circumstances. It is an object of the present invention to provide an optical pickup apparatus in which a focus offset is prevented from occurring and reading of informational signals written on a recording medium can be satisfactorily achieved without complicating the structure.
In order to overcome the above-mentioned problem and to accomplish the object, the present invention provides an optical pickup apparatus comprising a flat plate like transparent member having one side which is a polarization reflection surface and the other sides which is a total reflection surface, upon which light reflected from a recording medium which is irradiated with a flux of light is inclinatingly incident from the one side thereof; and a photo-detecting element for detecting the flux of light reflected by the one side and the other side of the flat plate transparent member which is irradiated with the flux of light.
As mentioned above, in the optical pickup apparatus of the present invention, light reflected from a recording medium which is inclinedly incident from one side upon a flat plate like transparent member having the one side which is a polarization reflection surface and the other side which is a total reflection surface is separated at the one side thereof into two fluxes of light depending upon the direction of polarization. Since the flux of light reflected on the one side and the flux of light which has transmitted through the one side and has been reflected on the other side of the transparent member are detected by a photo-detecting elements, the polarization direction of reflected light from the recording medium can be detected by comparing the intensity of these fluxes of light .